The present invention relates to an apparatus for performing solid-liquid separation by use of a filtration drum having a rotary drum and filter cloth mounted thereon.
U.S. Pat. No. 4,038,187, Japanese Patent Laid Open No. 49-80664, and Japanese Patent Laid Open No. 61-35886, for example, disclose a so-called rotary drum type solid-liquid separation apparatus wherein a solid-liquid mixture is introduced into a filtration drum having a drum body, and a filter medium, e.g., a stainless steel wire net or flocked filter cloth, mounted around the drum body, and the liquid is allowed to pass through the filter medium while the filtration drum is rotated, thereby separating the solid content from the solid-liquid mixture. Apparatuses of this type are used in a variety of industrial fields.
The stainless steel wire net used as the filter medium of the conventional solid-liquid separation apparatus is required to have a sufficient strength, and therefore, reduction in the wire diameter is restricted. If a wire net with small meshes is used, the treating ability of the apparatus is reduced due to the small open area ratio of the net. In order to enhance the treating ability using a wire net with small meshes, the rotational speed of the filtration drum or the level differential between the liquid inside and outside the drum needs to be increased.
However, if the rotational speed of the filtration drum is increased, the number of times the elements constituting the drum, particularly the reinforcing ribs, plunge into and emerge from the liquid increases correspondingly, with the result that the surface of the liquid inside the drum is caused to rise and fall violently. The solids caught by the filter medium will therefore be partly washed off before they reach a collecting position located at an upper portion of the drum. Thus, the amount of the solids that can finally be collected is still small, and the treating ability of the apparatus is not enhanced so much. On the other hand, in the case in which the level differential between the liquid inside and outside the drum is increased, it is necessary to use larger-sized or a greater number of reinforcing ribs, so as to withstand an increased mechanical load. This arrangement, however, causes the liquid surface to rise and fall even more turbulently. In the worst case, the filtration drum will vibrate due to the undulation of the liquid, preventing stable solid-liquid separation.
A wire net having small meshes may be used so as to catch small substances such as suspended substances. However, a problem arises in that the wire net becomes clogged in a short period of time. Once the net becomes clogged, the substances attached to the net cannot be removed easily even if the net is washed.
As the filter medium, flocked filter cloth is known which is formed by flocking short fibers of 30 to 100 .mu.m in diameter on the surface of a woven material by using an adhesive thereby to form raised fibers, as disclosed in the above-mentioned Japanese patent laid open No. 61-35886. Also known as the filter medium is napped filter cloth which is formed by napping a base woven material to form raised fibers or naps having a large diameter of several tens of microns. In these conventional filter cloths, the strength required as the filter cloth is imparted by the base woven material, and the raised fibers formed on the surface of the woven material serve to catch or arrest solids. Thus, the raised fibers serve as a filter layer. The apparatus using the conventional filter cloths are, however, poor in the solid arresting rate and the treating ability.
This is because, in the case of the flocked cloth formed with raised fibers by using an adhesive, the density of the fibers must not be so high that the openings of the woven material may not be filled with the adhesive, and because the diameter of the raised fibers is as large as 30 to 100 .mu.m and therefore the interval between the fibers cannot be made small. For these reasons, when the conventional flocked filter cloth is used, it is essentially necessary to combine several solids into a larger one by using a high-molecular coagulant etc., which leads to an increase in the running cost and causes the problem of toxicity which some coagulants have. Furthermore, the use of coagulant results in an increase in the amount of solids, and the amount of the coagulant to be added needs to be checked all the time, thus making the inspection of the operation conditions of the apparatus extremely troublesome.
The raised fibers with a large diameter are rigid and unyielding, and consequently the space between the fibers is large and deep. Further, the number of the raised fibers is so small that the surface of the woven material is visible, and, in some cases, the fibers become curled and make the filter layer bulky. Therefore, if solids enter the space between the raised fibers while being deformed, they will not easily come out of the space. As a result, the filter cloth becomes clogged and impairs the treating ability of the apparatus. Clogging of the filter cloth is particularly noticeable when the rigid fibers penetrate the solids.
Japanese Patent Laid Open No. 59-115720 and U.S. Pat. No. 4,707,260, for example, disclose filter cloth for solid-liquid separation, which comprises a belt-like base material and a filter layer formed by raising the fibers on the surface of the base material, and disclose a belt press type dewatering apparatus using the filter cloth. At least the weft extending in the width direction of the base material is formed of a polyester fiber. The raised fibers comprise fine fibers which are formed by napping chiefly the weft in one direction along the length of the base material and have a diameter of 0.1 to 10 .mu.m.
Thus, the filter cloth employed in this belt press type dewatering apparatus has raised fine fibers which are flexible and yielding, thus providing very narrow and relatively shallow space between the raised fibers. Therefore, the filter cloth can arrest even very small solids and at the same time be prevented from becoming clogged, whereby a solid arresting rate, treating ability, etc. much superior to those obtained with the flocked filter cloth mentioned above can be achieved.
In the rotary drum type solid-liquid separation apparatus, solids contained in the solid-liquid mixture are separated from the liquid as the mixture passes through the filter cloth. The solids caught by the filter cloth are successively collected or withdrawn by collecting water flushed onto the filter cloth. Thus, in the rotary drum type solid-liquid separation apparatus, the filter cloth is alternately used for separation of solids from the liquid and withdrawal of the separated solids. The filter cloth is therefore required to have excellent capability of arresting solids and releasing them when water is flushed thereto.
Further, the rotary drum type solid-liquid separation apparatus utilizes the level differential between the liquid inside and outside the filtration drum, for causing the solid-liquid mixture to pass through the filter medium. The filtration drum and the filter medium attached to the drum body therefore receive an external force corresponding to the level differential. To withstand the external force, the filtration drum is constituted by a rotary drum structure which includes a drum body having a plurality of annular stays arranged parallel to each other and a plurality of reinforcing ribs extending horizontally inside the stays and connecting the stays together, and a filter medium mounted around the drum body. The filter medium receives not only the external force corresponding to the above-mentioned level differential but also the pressure of water flushed when the arrested solids are collected and when the filter medium is washed thereafter. If the filter medium is made of a material having high strength, such as a stainless steel wire net, no problems will be caused. In the case of cloth filter, on the other hand, consideration must be given to the strength of the cloth in view of the forces applied to the cloth.
In the rotary drum type solid-liquid separation apparatus conventionally employed, the solids arrested by the filter medium are washed off by a collecting medium flushed from the nozzles of a collecting sprayer arranged at an upper portion of and inside the filtration drum, and then are discharged from the apparatus through a collecting hopper located inside the filtration drum. After the filter medium is washed by filtered water flushed from the nozzles of a wash sprayer arranged outside the filtration drum on the downstream side of the collecting sprayer with respect to the rotational direction of the drum, it is used again for solid-liquid separation. In another type of solid-liquid separation apparatus, washing-off of the solids from the filter medium and washing of the filter medium are carried out concurrently.
The collecting medium flushed directly to the arrested solids from the nozzles of the collecting sprayer for washing off the solids, however, sometimes pushes part of the arrested solids to the outside of the filter medium, i.e., to the side of the filtered liquid. The solids leaked out to the outer surface of the filter medium degrade the quality of the filtered water.
Part of the collecting medium is repeatedly circulated. The solid content in the collecting medium will therefore be broken into small pieces, which possibly clog the filter medium or mix with the filtered water and lowers the quality of the same. Furthermore, the circulation line of the collecting medium, particularly the nozzles of the collecting sprayer, will be clogged with such solids, causing inconveniences in the operation of the apparatus.
In the case of the solid-liquid separation apparatus wherein washing-off of the solids within the filtration drum and washing of the filter medium are performed at the same time, if filter cloth having a raised fiber layer thereon is used as the filter medium, the raised fiber layer facing the interior of the filtration drum can be damaged by pressurized water flushed from the sprayer. Thus, the filter cloth is degraded in the capability of arresting the solids in the solid-liquid mixture, and the service life thereof is shortened.
Furthermore, as the solid-liquid separation apparatus is operated for a long period of time, bacteria or the like may be propagated in the filter medium and clog the same, thus degrading the filtering ability of the apparatus. In such case, the filter medium may be washed using chemicals. To ensure that the chemicals dissolve and remove the extraneous matters, the filter cloth desirably has a property of retaining the chemicals for a long period of time.